Conventionally, various semiconductor materials are studied as materials for a radiation conversion layer of a highly sensitive radiation detector. Research and development have been made on single crystal bodies of CdTe (cadmium telluride) or CdZnTe (zinc telluride cadmium), and some products have been commercialized. When radiation is incident on these single crystal bodies, carriers (electron-hole pairs) corresponding to the dose of radiation will be generated, and the radiation is detected by taking out charge signals resulting from these carriers. However, for application to a radiation detector for medical diagnosis, it is necessary to form a radiation conversion layer of large area (e.g. 20 cm square or larger). To form a single crystal body of such a large area is technically difficult and requires huge cost. Then, a radiation conversion layer consisting of a crystal growth as a polycrystal compound semiconductor layer instead of the single crystal body is considered promising.
With a small radiation detector using CdTe single crystal, it is known that doping CdTe single crystal with Zn (zinc) reduces leakage current flowing through CdTe single crystal. Doping CdTe single crystal with a halogen such as Cl (chlorine) is known to improve carrier mobility in CdTe single crystal. For example, an optimal Cl concentration for improving detection performance of CdTe single crystal is disclosed in Patent Document 1.
As for factors obstructive to the carrier mobility in the single crystal body of CdTe, a charge trap due to crystal defects or impurities is considered to be a main factor. However, with the polycrystal compound semiconductor layer, it is thought that the action of crystal grain boundaries and grain size distribution is added, and the optimal Cl concentration is not necessarily the same as in the case of the single crystal body.
An optimal Zn concentration for doping the CdTe polycrystal compound semiconductor layer, although disclosed in Patent Document 2, for example, has a wide range of several to several tens of mol %. Although its doping has an effect such as of reducing leakage current, its extent has been unknown. Optimal Zn concentration and Cl concentration when doping with Zn and Cl have been unknown. Thus, the optimal concentrations of Zn and Cl for doping the polycrystal compound semiconductor layer of CdTe have been unknown.